In a magnetic recording device in which a read head comprises a magnetoresistive (MR) sensor, there is a constant drive to increase recording density. One trend used in the industry to achieve this objective is to decrease the size of the MR sensor. Typically, the sensor stack has two ferromagnetic layers that are separated by a non-magnetic layer. One of the ferromagnetic layers is a reference or pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent antiferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer with a magnetization that rotates in response to external magnetic fields, and is aligned either parallel or anti-parallel to the magnetization in the pinned layer to establish a “0” or “1” memory state. When an external magnetic field is applied by passing the MR sensor over a recording medium at an air bearing surface (ABS), the free layer magnetic moment may rotate to an opposite direction. A MR sensor may be based on a tunneling magnetoresistive effect where the two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. A sense current is used to detect a resistance value which is lower in a “0” memory state than in a “1” memory state. In a CPP configuration, a sense current is passed from a top shield through the sensor layers to a bottom shield in a perpendicular-to-plane direction.
In a longitudinal biasing read head design, hard bias films of high coercivity are abutted against the edges of the sensor and particularly against the sides of the free layer. In other designs, there is a thin seed layer between the hard bias layer and free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for sensor elements become smaller with higher recording density requirements, the free layer becomes more volatile and more difficult to bias. Traditional biasing schemes using a hard magnet bias have become problematic due to randomly distributed hard magnetic grains within the hard bias layer.
In recent years, 2DMR and 3DMR configurations have become attractive from an areal density improvement standpoint. However, shield stability is more difficult to control in 2DMR and 3DMR schemes because of a requirement to shrink reader-to-reader spacing (RRS) and in view of repeated thermal treatments during fabrication that can readily flip the magnetization in the shields. Although an upper shield that is stabilized through antiferromagnetic (AFM) coupling provides good thermal stability in a single reader (1DMR) structure, repeated heat treatments on shields in a 2DMR or 3DMR process flow greatly increase shield instability and flip rate. In addition, shields that are stabilized with exchange coupling become less stable due to the reduced RRS requirement. Since top and bottom shields are commonly directly coupled or anti-ferromagnetically coupled to the junction shield that biases a free layer in the MR sensor, shield instability will directly translate into reader instability and will adversely impact signal to noise ratio (SNR) and bit error rate (BER). Accordingly, a new read head structure is needed wherein shield stability is improved in 2DMR and 3DMR configurations while maintaining acceptable SNR and BER.